There has been a need for a tough but flexible polyurethane elastomeric composite material that can be used to make articles such as hydraulic cylinder seals that have excellent abrasion resistance, low compression set, and good thermal stability, oil resistance and tensile properties. It has been desired that such a material be injection moldable to be able to form articles having intricate shapes and stringent dimensional tolerances. It is further desired that this material be thermoplastic in nature to facilitate ease and economy in processing and minimization of scrap waste.
Polyurethane is formed by reacting a polyol with an isocyanate in the following manner: ##STR1##
The overall properties of the polyurethane will depend upon the type of polyol and isocyanate, the degree of crosslinking and crystallinity in the polyurethane, the molecular weight of the polyurethane and chemical structure of the polyurethane backbone.
Polyurethanes may be either thermoplastic or thermoset, depending on the degree of crosslinking present. Thermoplastic urethanes do not have primary crosslinking while thermoset polyurethanes have a varying degree of crosslinking, depending on the functionality of the reactants.
Thermoplastic polyurethanes are commonly based on either methylene diisocyanate (MDI) or toluene diisocyanate (TDI) and include both polyester and polyether grades of polyols. Further, both thermoset and thermoplastic polyurethanes can be formed by a "one-shot" reaction between isocyanate and polyol or by a "pre-polymer" system, wherein a curative is added to the partially reacted polyolisocyanate complex to complete the polyurethane reaction. Examples of some common thermoplastic polyurethane elastomers based on a "pre-polymers" are "Texin", a tradename of Miles, Inc., "Estane", tradename of BFGoodrich Co., "Pelletbane", a tradename of Dow Chemical Co., and "Elastollan", a tradename of BASF, Inc.
Several 1,5 naphthalene diisocyanate (NDI) based thermoset polyurethanes are commercially produced. One example of such a thermoset polyurethane is "Vulkallon", a tradename of Miles, Inc. However, thermoset polyurethanes cannot be used to injection mold intricate articles with tight dimensional tolerances. Further, any scrap generated during the molding of a thermoset polyurethane cannot be reused whereas the scrap from molding a thermoplastic polyurethane can be ground, dried and reused.
U.S. Pat. No. 4,254,010 issued to S. Mizuno et al. on Mar. 3, 1981, proposes the use of NDI as one of several isocyanates that may be alternatively used to form a rigid (non-elastomeric) thermoplastic polyurethane plastic material. Heretofore, there has been no known thermoplastic polyurethane elastomer that is based on NDI.
Hydroquinone bis 2-hydroxyethyl ether (HQEE) has been used as a chain extender in pre-polymer type MDI and TDI (non-NDI) based polyurethanes. Heretofore, there has been no known thermoplastic polyurethane elastomer that is based on HQEE and NDI.
The use of glass fibers in polyurethane compositions is described in U.S. Pat. No. 4,254,010, issued to S. Mizuno et al. on Mar. 3, 1981. That patent discloses the dispersal of aminosilane or epoxysilane coated glass fibers in a thermoplastic polyurethane to make a resin composition suitable for forming rigid (plastic) articles that resist warpage during molding. Heretofore, there has been no known thermoplastic polyurethane elastomeric composition containing fibers that has superior wear and compression set characteristics but still remains elastic enough to be used as a seal compound.
The present invention is directed to overcoming one or more of the problems as set forth above.